1. Field of the Invention
The present invention relates to a fuel control apparatus for an internal combustion engine.
2. Discussion of Background
FIG. 1 shows schematically the construction of an electronic control device for an internal combustion engine. In FIG. 1, a reference numeral 1 designates an air cleaner, a numeral 2 designates a hot wire type air flow sensor, a numeral 3 indicates an intake air temperature sensor for detecting the temperature of sucked air, a numeral 4 represents a throttle valve disposed in an air intake pipe to there-by control an amount of air to be sucked into the engine 16, a numeral 5 a throttle valve opening degree sensor which is connected to the throttle valve 4 to detect a degree of opening of the throttle valve, a numeral 6 a surge tank, a numeral 7 a bypass air quantity adjusting valve disposed in an air passage 14 which bypasses the upstream side and the downstream side of the throttle valve 4, a numeral 8 an intake manifold, a numeral 9 a water temperature sensor attached to a cooling water passage in which cooling water for cooling the engine 16 flows, a numeral 10 an injector attached to each cylinder, a numeral 11 an air intake valve driven by a cam (not shown), a numeral 12 a cylinder, a numeral 13 a crank angle sensor for detecting a crank angle and the revolution speed of the engine 16 and a numeral 15 an electronic control unit (ECU).
The operation of the conventional fuel control device will be described.
The ECU 15 calculates a fuel supply quantity to the engine on the basis of an intake air quantity detected by the air flow sensor 2, a crank angle signal generated from the crank angle sensor 13 and a cooling water temperature detected by the water temperature sensor 9, and controls the injector 10 to inject fuel in synchronism with the crank angle signal. The outputs of the intake air temperature sensor 3 and the throttle valve opening degree sensor 5 are used-as auxiliary parameters. The ECU 15 also controls the bypass air quantity adjusting valve 7. However, the details of the operation concerning the control of the adjusting valve 7 are omitted.
The calculation of the intake air quantity by the ECU is conducted in such a manner that the intake air quantity Q detected by the air flow sensor 2 is sampled at constant time intervals and the mean value Q.sub.A of the sampled intake air quantities is obtained in synchronism with a leading edge (or trailling edge), for instance, a point B, of a crank angle signal. In other words, the mean value Q.sub.A of the intake air quantities is obtained in the period between adjacent leading edges, such as points A and B, of the crank angle. Namely, ##EQU1## Thus, a fuel quantity to the engine was obtained on the basis of the value.
Since the above-mentioned conventional apparatus operates to calculate the fuel quantity to the engine on the basis of the mean value of intake air quantities sampled between given crank angles, the period of a crank angle signal becomes short when the engine is operated at a high revolution speed as shown in FIG. 3. This results in the reduction of the number of samplings of the intake air quantity. Accordingly, even when each intake air quantity to the engine is constant at a steady state, an intake air quantity Q.sub.AD calculated at a point D and an intake air quantity Q.sub.AE calculated at a point E respectively have values Q.sub.10 /1 and Q.sub.20 /1; thus the values Q.sub.AD and Q.sub.AE are different from the actual intake air quantity. This is because the number of samplings is too small with respect to a crank angle period. In order to assure a sufficient number of samplings, it can be considered that a period of calculating a crank angle should be 2 or 3 times as long as the crank angle signal period. In this case, however, there is a problem of poor response because the number of samplings is too great when the engine is operated at a low revolution speed.